2. Rare earth-transition metal systems
2.1.7.3. Crystalline compounds
2.1.7.3.1. Lanthanum-nickel systems
Gasgnier (1981) prepared LaNi films by thermal evaporation. Electron diffraction measurements of the as-deposited samples show the coexistence of small crystallites of Ni and an amorphous alloy. After annealing with the electron beam, Ni recrystallizes, and a second ring pattern appears which can be indexed according to a hexagonal structure with: a = 3.95 and c = 6.90.~. By further annealing, new ring patterns appear, but they are complex. Only one of them was indexed as a fcc lattice with a = 7 ,~ (bulk LaNi: is fcc with a = 7.35 .~). So, it was not possible to link the thin film structures to ones in the bulk state. Also for CeNi films unknown structures were observed.
54 M. GASGNIER 2.1.7.3.2. Samarium-nickel systems
As mentioned previously for GdFe and GdCo, the films studied by Buravikhin et al. (1975a) are not strictly amorphous. They reported the magnetization as a function of Sm contentmit decreases rapidly to zero for 22at% Sm--and magnetostriction (near 0) for SmNi2.
The formation of a number of crystal structures were observed as the Sm concentration varied for SmNi samples deposited at 470K, and annealed at 740K: Ni and Sm (15%), Ni, Sm2Ni17, SmNi2 and SmNi~ (15-40%), Ni and SmNi2(40-60%) and SmNi2 and SmNi (60-70%) have been observed by Buravikhin et al. (1973). They reported 4zrMs before and after annealing and gave some hysteresis loops. They showed that Sm2Ni17 has a higher magnetization than Gd2Ni17. They conclude that Sm is less active than Gd in forming inter- metallic compounds (see references listed by these authors). Kameneva et al.
(1976) studied the magnetic properties of SmNi films as a function of the Ni content and temperature. They showed that such materials can have a super- paramagnetic behavior.
2.1.7.3.3. Gadolinium-nickel systems
Buravikhin et al. (1975a) observed a more rapid decrease of the magnetization for GdNi than for SmNi--it reaches zero at ~ 16 at% Gd; the magnetostriction of GdNi is lower than for GdFe and GdCo and decreases rapidly to zero for 32 at%
Gd as in GdNi2.
Egorov et al. (1974), by simultaneous thermal evaporation (tungsten crucibles), studied the formation of various compounds in GdNi films deposited at 470 K and heated at 720 K. They observed, depending on the Gd content, the presence of several phases such as Gd2Ni17, GdNi4, Gd2NiT, GdNi3, GdNi2 and GdNi.
Domyshev et al. (1974) reported the magnetic, magnetostriction and magneto- elastic properties for GdNi2 compounds. They measure a Tc of 140 K, and low magnetostriction, 4~-Ms, H~ and Hk values compared to bulk GdCo2. As men- tioned above, Kamena et al. (1976) attributed a superparamagnetic behavior to GdNi films. They reported numerous data such as H~, 47rM0, hysteresis loops and magnetic domains as a function of the Gd content, the annealing temperature and the Ni grain size. They observed that the magnetic domains vanish when the Gd content increases. This may be due either to a decreasing magnetization or to a more strongly superparamagnetic behavior. After annealing they noticed a higher magnetization which is interpreted by an increase of the Ni grain sizes.
2.2. Rare earth-(3d)--4d transition metal systems 2.2.1. Gadolinium-niobium alloys
To our knowledge McGuire and Gambino (1980) are the only ones to have reported some magnetic properties for such systems. (For a comparison with other elements see sections 2.1, 3.3, 7.1, 7.2 and 7.3.) They have shown that these alloys have lower Tc (48 K), 0 (90) and pr~/p (-0.012) values.
RE ALLOYS AND COMPOUNDS AS THIN FILMS 55 2.2.2. Gadolinium-cobalt-molybdenum alloys
2.2.2.1. Main magnetic properties
As mentioned by Hasegawa et al. (1975a) and Chaudari and Cronemeyer (1976), the addition of Mo to GdCo amorphous alloys decreases the magnitude and temperature dependence of the magnetic moment. This metal has a low density of states at the Fermi level, behaves like a nonmagnetic metal, and strongly reduces the moment of the Co sublattice. The preparation of ternary alloys has been done to produce bubble domain devices; other diluents like Au and Cu have been proposed (see sections 3.2 and 3.3). It should be recalled that Eschenfelder (1980) has reported numerous data about magnetization and bubble properties for these materials.
Ferrimagnetic alloys such as (Odl-xCo~)l_yMoy prepared by rf sputtering have been studied and discussed by Hasegawa (1975), Hasegawa et al. (1975a,b), Chaudari et al. (1975), Cuomo and Gambino (1975), Argyle et al. (1975) and Kobliska et al. (1975a). They have reported the main magnetic data and sug- gested several models to explain the film properties. Several authors have shown that sputtering parameters and geometry influence the film uniformity and that substrate temperature and nature influence the film properties (Cuomo and Gambino, 1975; Chaudari et al., 1975; Kobliska et al., 1975b; Bajorek and Kobliska, 1976). Heiman et al. (1978) have studied the effects of substrate bias and observed that Mo is largely unaffected by Vb (the Mo content remains constant).
Some other results can be reported. Hasegawa et al. (1975a) have shown the variation of Tc and Tcomp versus the Mo content and temperature, and have offered a band model based on a simple molecular field analysis of their magnetization data. They also gave some experimental results obtained by the polarization Kerr effect. Argyle et al. (1975) have concluded that Kerr rotation is due mostly to Co atoms. McGuire et al. (1977) have reported the transport properties (Hall resistivity, Hall coefficient, resistivity) and the magnetic proper- ties (magnetization) for an alloy such as GdT~ColsMOll versus temperature.
Above Tcomv (~230 K), Pn and Rs are positive. The authors discuss the results in terms of inhomogeneities and of impurities (02, argon . . . . ) which can increase the sample resistivity. Mizoguchi et al. (1978) have studied the in-plane suscep- tibility near Teomp (~200 K) for two different samples. The susceptibility presents a minimum value at Tcomp. They developed a theory of the in-plane susceptibility in terms of an antiferromagnetic or a columnar susceptibility, They discussed the Ku and remanent magnetization dependence on temperature. They compared experimental and theoretical results in an attempt to explain the origin of the susceptibility, and indicate the influence oxygen can have upon the anti_fer- romagnetic susceptibility. They conclude that the in-plane susceptibility appears to rule out the possibility of a micromagnetic columnar anisotropy and that the films are highly uniform except for a thin surface layer. They observe that Ku is 1.7 times higher when a low evaporation rate is used than that obtained at in high rates; this result is due to a lower Mo content. They noticed that argon has no effect on Ku. By comparison with evaporated films the authors noted a larger
56 M. GASGNIER
in-plane anisotropy than by bias-sputtering which is thought to be due to inhomogeneities.
Such materials are convenient to support bubble domains. In order to produce suitable devices numerous experiments have been done. Kryder et al. (1974, 1975, 1976a,b, 1977), Chaudari et al. (1975), Hasegawa (1975), Bajorek and Kobliska (1976), Hafner and Humphrey (1977), Kobliska et al. (1977) and De Luca et al. (1978) have studied the bubble mobility (propagation, expansion, velocity) dependence on an overlayered permalloy film such as NiFe, or on an in-plane driving field or on a pulsed current, or on laser impact, for different kinds of structured devices (T-I and Y-I bars and chevrons). Chaudari et al.
(1975) have observed that for a well-defined temperature dependence of the bubble device the magnetization should be as nearly constant as possible near the operating point of the device and Tc and Tcomp must be far from 293 K; for GdCoMo alloys the best behaviour versus temperature is obtained on the Co-rich side with 12-15 at% Mo. They showed that Mo lowered the Co moment, and determined the magnetic properties of these specimens. One of the main characteristics is reached for a content of 16 at% Mo: the product AQa 4~rMs has the smallest value. Kryder et al. (1976b, 1977) have reported a bubble velocity of 500 m/s; Hafner and Humphrey (1978) give an instantaneous velocity of 400m/s. De Luca et ai. (1978) have studied the propagation of magnetic domains (stripe domains and bubble raft as a hexagonal array of bubbles) with a current pulsed directly through the film; they discuss the propagation depen- dence on the Gd content. Such bubble raft configurations and stripe domains have been observed by Chaudari and Herd (1976) and Herd (1976) by Lorentz microscopy (see section 2.3).
2.2.2.2. Effect of rare gas incorporation
Bajorek and Kobliska (1976) have reported that excess Ar and Mo influence the magnetization. They concluded that this primarily depends on film com- position whereas the anisotropy energy density exhibits a more complex behavior. Cuomo and Gambino (1977) have shown that large rare gas (Ne, At, Kr) concentrations induce metastable quaternary alloys such as GdCoMoAr.
They believe that the amorphous alloys can accommodate relatively high concen- trations of large interstitial voids in the disordered materials. Their samples are stable from 293 K to the crystallization point. They conclude that the magnetic dilution provided by the gas produces a low 4zrM~; therefore these alloys are suitable for bubble domain applications. Kobliska et al. (1977) have studied the change in magnetic properties for alloys with two different Ar contents inducing bubbles with either 2/xm or 1/xm diameter sizes. They discussed and computed the optimum values of Q and w~ and the optimum composition with saturation magnetization at 290 K for acceptable temperature sensitivity. They deduced that an ideal material should have Gd/Co = 0.095, 47rM~ = 1500 G and T~ = 450 K. They concluded that the degree of temperature insensitivity improves with decreasing bubble diameter and is satisfactory only for sub-half-micron bubble materials.
RE ALLOYS AND COMPOUNDS AS THIN FILMS 57
Mizoguchi et al. (1977a) have studied such alloys of various compositions obtained by bias voltage variations. They showed how Ar + ion implantation can damage the magnetic properties and noted that Ku decreases almost linearly with doses up to 10 TM Ar+/cm: and decreases more slowly at higher doses. They used a short-range atomic ordering model to explain the anisotropy. Gangulee and Kobliska (1978a) investigated the temperature dependence on the saturation magnetization of (GdCoMo)91Ar9 films in terms of the mean-field theory. They gave. spin and exchange interaction energy values for different Ar and Gd/Co contents, subnetwork magnetization, g, A and Ku variations versus temperature.
They observed an increase of the dipolar coupling constant with increasing Ar content, while it appears that no correlation with the magnitude of Ku exists.
They concluded that Ku does not only depend on the composition but also on the manufacturing parameters, and that the temperature dependence of the aniso- tropy energy can be described in terms of a dipolar model. The same authors (1978b) compared the same magnetic properties for two specimens with different Ar contents. They observed a change in saturation magnetization, a decrease for Tc and Tcomp with increasing Ar content (T~omp went from 360 K to 155 K, for amounts of Ar increasing from 2 to 9at%). They noted that the exchange coupling constants values do not vary, except the Gd-Co one, which decreases with increasing Ar content. Nishida et al. (1979) report Ku and 4~rMs variations at 293 K versus at% Mo and Vb, and the Ku and A dependence after annealing at 500 K on the heating time. They suggested that a distribution of atomically ordered pairs in ternary alloys accounts for this behavior. As Heiman et al.
(1978), Cuomo and Gambino (1975) and Gangulee and Kobliska (1978a,b) showed that the Mo atoms are hardly resputtered from the film (this leads to a rapid decrease of KO and such materials have a good thermal stability of the anisotropy. Nishida et al. (1979) suggested that a Mo atom is strongly tied to its neighboring atoms and suppresses movement of Co atoms and this changes the direction of the Co-Co pair axis (see section 3.1 for a comparison with GdCoCu alloys).
Mizoguchi and Cargill (1979) give many details about Ku which arises from magnetic dipolar interactions involving anisotropically distributed atomic moments or anisotropic microstructures. They describe also anisotropy aris- ing from pseudo-dipolar anisotropic exchange; they demonstrate that clas- sical dipolar interactions can produce magnetic anisotropy for Q > I , and compared the structural anisotropies for both sputtered and evaporated samples.
2.2.2.3. Effect of oxygen and hydrogen incorporation
Argyle et al. (1975) have determined the total magnetization and Co sublattice magnetization dependence on temperature from Kerr rotation measurements, for alloys prepared by rf sputtering. They observed anomalous polarization Kerr loops due to interface effects relative to the Gd oxidation at the surface. They concluded that the formation of a Gd203 layer results in an enrichment of Co in the top (film-air interface) with a thickness of 50-100 ]k. In order to have a
58 M. GASGNIER
complete magnetic analysis they removed this layer or protected the as- deposited films by SiO2.
Malozemoff et al. (1977), by using polar Kerr effect (as 0) and ellipticity (as e) measurements of the reflected light which arises from magnetic circular di- chroism, concluded to the formation of a surface layer due to oxidation. It gives a contribution which adds to the bulk 0 but subtracts from the bulk ~. They removed this surface by etching. The film-substrate interface showed that 0 and values are different from the bulk. With increasing Mo content 0 and e both drop, while with increasing Gd and O content 0 does not change but E increases.
They discussed the degree of surface oxidation.
More recently Hafner and Hoffmann (1979), Hafner et al. (1980) and Stobiecki et al. (1980) have studied, either by ageing or by annealing with and without oxygen (or hydrogen), the magnetic properties and the depth magnetic profiles of their samples prepared by rf sputtering. As for Gd-Co films, they have established a columnar model to explain the film growth, and 02 diffusion along the boundaries of the columns. In this way there is formation of Gd203. They proposed two models for the magnetization behavior before and after annealing of film~ with different compositions. From susceptibility measurements they determined 47rMs, Hk, H~ and Ku changes at the free surface and at the film-substrate interface. These values depend also on the thickness: they are larger for thicker films. On the other hand they have observed that the effect of annealing with an 02 partial pressure destroyed the perpendicular anisotropy while annealing at 570 K in a 10 -6 Torr vacuum leads to a stable perpendicular anisotropy. With a partial pressure of oxygen or of hydrogen the magnetization increased. These experiments show that Hk and Ku increase with annealing and that Q has a maximum at 570 K.
Stobiecki and Hoffmann (1980) have reported the influence of oxygen on the transport properties for two kinds of films, one with Tcomp above 293 K and one with Tcomp below 293 K. From p, EHE, and Tcomp measurements they concluded that two effects are observed: ordering, especially in the Gd sublattice, and oxidation. Below 520 K usually ordering effects dominate. Above 520 K oxygen diffuses (along column boundaries, as mentioned above) into the film and decreases the net Gd sublattice magnetization. The Co and Gd sublattices contribute then to the Hall effect. In this way they refute the results of Shirakawa et al. (1976) who suggested a dominant effect of the Co sublattice magnetization from their experiments on E H E and polar Kerr rotation by annealing from 4 to 525 K; but they fit in with those of McGuire et al. (1977).
2.2.2.4. Effects of annealing
The temperature dependence of the magnetic properties of these alloys has been studied by different authors but they have not determined the role of oxygen. Kobliska and Gangulee (1975) have shown that a sample such as Gd15Co70Mo15 retained all functional magnetic properties even after annealing at 620 K. However, some changes are accounted for, particularly a shift of Tcomp from 230 to 250 K after annealing from 420 to 620 K. These are believed to be due to structural and/or stress changes. Chaudari and Cronemeyer (1976) have
RE ALLOYS AND COMPOUNDS AS THIN FILMS 59 reported the variations of Hu, 4zrMs, Ku and the g-factor from 80 to 570 K. Their rf sputtered samples have a Tcomp below 293 K. They noticed the presence of a strong surface mode removed by ion milling. They described a model to determine the subnetwork moments which are related to the anisotropy energy.
They concluded that Ku may be made to fit a pseudo-dipolar variation of the subnetwork magnetization. But the values of the coupling constants (Gd-Co being dominant) and their surprising behavior with Mo additions (large varia- tions) do not readily explain the anisotropy in a real physical situation.
Cuomo and Gambino (1977) have reported 4~rMs variations versus tem- perature from 4 to 580 K for two different alloys with Ar and Kr incorporation.
Both samples show the same Tcomp (190 K) and the same T, (500 K), but at 0 K, 47rM~ is lowered from 1550 to about 1300 Oe for the specimen with the higher gas content. Heiman et al. (1978) found no significant change in 4~-M or Ku by annealing. They concluded that Mo stabilizes the films against the annealing effects. By comparison with Cu or Au additions for which increases are found (see sections 3.1 and 3.3) they suggest that the differences are due to the chemical bonding: Cu and Au are insoluble in Co, while Mo forms many compounds with Co. Katayama et al. (1978) have shown changes of Ms and Ku with annealing time at a temperature of 500 K and for different Mo contents.
They concluded that Mo improves the chemical stability of the films and that the rates of change of Ms and K~ decrease with increasing Mo concentration. They also reported the following parameters for a sample with 14 at% Mo: A, 2w, Crw and I. In XPS studies they did not observe a shift of the 4f band.
Stobiecki et al. (1980) found that annealing at 770 K leads to a crystallization state (microcrystalline structure) which destroys the perpendicular anisotropy.
Cuomo and Gambino (1977) reported a crystallizationtemperature of 1070 K for alloys with 15 at% Mo in a vacuum of 10 -9 Torr. Hafner et al. (1980) showed that proper annealing leads to a magnetization maximum at 570 K and that at these temperatures Gd and Co are oxidized.
Remark. Discussion about Ko has been mentioned for GdCo alloys (see section 2.1.6.1).
2.2.2.5. Effects of rare earth substitution
Bochkarev et al. (1979) have pointed out that the magnetic properties of GdCoMo films may be changed by partially substituting Y and Sm for Gd to fabricate bubble domain devices. Their samples were prepared by sputtering.
They showed the evolution of 4~rM, Hcoll and P0 (domain structure period) for GdCo, YGdCoMo and YSmGdCoMo films. For the quinternary composition the three parameters do not change with temperature (290 to 420 K). They also reported 47r Ms, Ha, Q, P0, l and Ku values for different compositions.
2.2.2.6. Structure determined by electron and X-ray diffraction
Graczyk (1978) has investigated the short range order structure, micro- structure and compositional fluctuations of thin films (such as GdCoMoArO) prepared by bias sputter. From his small angle scattering experiments he reports the presence of one halo which shifts with lhe rotation angle and decreases in intensity as this parameter increases. This clearly shows the anisotropic scatter-
60 M. GASGNIER
ing. Bright field electron micrographs indicate both typical atomic adsorption, increasing the surface roughness, as well as the presence of voids. Another micrograph shows inhomogeneities with a cylindrical shape nearly perpendicular to the surface. For this alloy the author has calculated the small angle scattering curves for a low density void model and for a compositional fluctuation model, and he believes that the halo is partially due to interparticle interference.
Heiman et al. (1978) have observed that X-ray diffraction patterns exhibit little dependence on Vb and Ku; all their patterns are similar to those observed for GdCo amorphous films prepared with Vb = -100 V.
2.3. Lorentz microscopy: magnetic domains
The visualization of magnetic domains by electron microscopy is limited by the thickness of the films (about 1200 A). Grundy (1977, 1980) has written two review articles in which he describes and compares the properties of magnetic materials which are able to support bubble domains. He also gave numerous details relative to the observation of magnetic domains in the electron micro- scope.
2.3.1. Binary alloys
2.3.1.1. Neodymium-cobalt films
Zdanowics et al. (1978) have made some observations for Nd-Co amorphous films obtained by vacuum evaporation. The typical domain patterns can change with film enrichment with Co or with Nd. Co-rich films show zig-zag shaped
domain walls, Nd-rich films have a weaker magnetic anisotropy.
Nowak and Scharff (1979) have shown the existence of a primary ripple structure due to small crystallite sizes (40 Ik) and a large uniaxial anisotropy field in Nd35Co65 polycrystalline films deposited by vacuum evaporation. The samples have a columnar structure. In magnetic fields only the ripples rotate as a whole.
2.3.1.2. Gadolinium-iron films
Gill et al. (1978), for in-situ ion beam sputtered and rf sputtered amorphous GdFe alloys, have shown that films possess in-plane magnetization ripples, cross- tie walls and stress-induced magnetic anisotropy. Upon heating with the electron beam the domains change and there is formation of a radial cross-tie structure and circularly shaped ripples. Gasgnier and Colliex (1979) and Gasgnier et al.
(1979) have shown some magnetic structures such as stripe domains with a possible nucleation of bubbles for a sample with 25 at% Gd (near the compen- sation point) and of domain walls for as-deposited amorphous films prepared either by thermal evaporation or by e-gun. They have observed that the application of the magnetic field of the objective lens can turn or displace the domains and reverse the contrast of the domains, and that localized annealing as brief "pulse annealing" can create new domains which can develop over the ~ entire area of the film (fig. 4). Buravikhin et al. (1975, 1976) have reported some domain structures for GdFe amorphous and crystalline films prepared by ther-